U.S. Pat. No. 5,886,822 discloses an ophthalmic lens presenting a projection insert. Such a projection insert is constituted by an optical imager for shaping light beams delivered by an electronic and optical system for generating light beams from an electronic signal, of the miniature screen, laser diode, or light-emitting diode (LED) type. The optical semiconductor directs light beams towards the eye of the wearer in order to enable information content to be viewed.
An example of a prior art display is shown in its environment in greater detail in FIG. 1.
By way of example, the optical imager is of the same type as described in above-mentioned U.S. Pat. No. 5,886,822.
An electronic signal carrying information is brought to a miniature screen 1 by a cable 7. On the basis of this signal, the miniature screen 1, illuminated by a background projector 2, generates a pixel image corresponding to the information. By way of example, it is possible to use a “Kopin Cyberdisplay 320 color” screen generating images comprising 320×240 pixels and having dimensions of 4.8 millimeters (mm)×3.6 mm. The screen 1 is held by a mechanical interface 3 in position relative to the optical imager 5. A protective shell 4 protects all or part of the assembly.
The optical imager 5 comprises a propagation prism 5a, a counter prism 5b, a quarterwave plate 5c, and a spherical Mangin mirror 5d. The spherical Mangin mirror is a plano-spherical lens whose spherical face has been made reflecting by aluminum plating treatment or the equivalent.
The imager 5 also includes polarization separator treatment 6 which may be implemented in the form of a deposit of thin layers either on the propagation prism 5a or on the counter prism 5b, or by means of a film stuck between the two above-mentioned elements.
The imager 5 is embedded in a molded lens 9 and the housing is disposed on one side of the lens 9. On the same principle, the housing could be placed facing forwards at the back of the lens, an injection element forming a 90° reflector element then being included in the lens.
The word “lens” is used in particular for an optionally correcting lens suitable for mounting in a frame for eyeglasses. This ophthalmic lens presents conventional functions such as correcting eyesight, and coatings against reflection, dirtying, scratching, etc.
In order to enable focusing to be adjusted so as to adjust the viewing distance of the information image and consequently adjust the image so that the wearer can see it in sharp and comfortable manner, the lens 9 is secured to a plate 10 carrying two rods 10A and 10B on which the housing 4 is engaged in slidable manner. During this adjustment, the housing is engaged manually to a greater or lesser extent on the slide rods to adjust the distance D, and thereafter it is held in position by a transverse screw (not shown).
That type of device for adjusting focusing raises the following technical problems.
Depending on how it is adjusted, the overall size of the display varies. This can lead to constraints in its design.
This type of device inevitably leads to a considerable amount of slack between the slide rods and the housing, which harms the accuracy of the display.
The housing is generally made of plastics material and the plate 10 together with its rods 10A and 10B are made of metal. This results to the housing wearing quickly and further increasing the slack and lack of precision of the display.
Finally, in that prior art, the adjustment is performed manually in the sense that the wearer needs to take hold of the housing and cause it to slide along the rods until proper focus is obtained. Naturally, such an arrangement is not very accurate and is not very comfortable in use, being poorly adapted to everyday use that is not experimental.